refactor(material/form-field): tokenize density overrides

Switches the custom density implementation of the form field to be based on tokens.

Author: crisbeto

src/material/core/tokens/m2/mat/_form-field.scss

@@ -1,6 +1,9 @@
+@use 'sass:math';
 @use 'sass:map';
+@use '@material/textfield' as mdc-textfield;
 @use '../../token-utils';
 @use '../../../style/sass-utils';
+@use '../../../theming/theming';
 @use '../../../theming/inspection';
 @use '../../../theming/palette';
 
@@ -86,7 +89,53 @@ $prefix: (mat, form-field);
 
 // Tokens that can be configured through Angular Material's density theming API.
 @function get-density-tokens($theme) {
-  @return ();
+  $density-scale: theming.clamp-density(inspection.get-theme-density($theme), -4);
+  $size-scale: (
+    0: 56px,
+    -1: 52px,
+    -2: 48px,
+    -3: 44px,
+    -4: 36px,
+  );
+  $height: map.get($size-scale, $density-scale);
+  $hide-label: $height < mdc-textfield.$minimum-height-for-filled-label;
+
+  // We computed the desired height of the form-field using the density configuration. The
+  // spec only describes vertical spacing/alignment in non-dense mode. This means that we
+  // cannot update the spacing to explicit numbers based on the density scale. Instead, we
+  // determine the height reduction and equally subtract it from the default `top` and `bottom`
+  // padding that is provided by the Material Design specification.
+  $vertical-deduction: math.div(mdc-textfield.$height - $height, 2);
+
+  // Note: these calculations are trivial enough that we could do them at runtime with `calc`
+  // and the value of the `height` token. The problem is that because we need to hide the label
+  // if the container becomes too short, we have to change the padding calculation. This is
+  // complicated further by the fact that filled form fields without labels have the same
+  // vertical padding as outlined ones. Alternatives:
+  // 1. Using container queries to hide the label and change the padding - this doesn't work
+  // because size container queries require setting the `container-type` property which breaks
+  // the form field layout. We could use style queries, but they're only supported in Chrome.
+  // 2. Monitoring the size of the label - we already have a `ResizeObserver` on the label so we
+  // could reuse it to also check when it becomes `display: none`. This would allows us to remove
+  // the three padding tokens. We don't do it, because it would require us to always set up
+  // the resize observer, as opposed to currently where it's only set up for outlined form fields.
+  // This may lead to performance regressions.
+  // 3. Conditionally adding `::before` and `::after` to the infix with positive and negative
+  // margin respectively - this works, but is likely to break a lot of overrides that are targeting
+  // a specific padding. It also runs the risk of overflowing the container.
+  $filled-with-label-padding-top: 24px - $vertical-deduction;
+  $filled-with-label-padding-bottom: 8px - $vertical-deduction;
+  $vertical-padding: 16px - $vertical-deduction;
+
+  @return (
+    container-height: $height,
+    filled-label-display: if($hide-label, none, block),
+    container-vertical-padding: $vertical-padding,
+    filled-with-label-container-padding-top:
+      if($hide-label, $vertical-padding, $filled-with-label-padding-top),
+    filled-with-label-container-padding-bottom:
+      if($hide-label, $vertical-padding, $filled-with-label-padding-bottom),
+  );
 }
 
 // Combines the tokens generated by the above functions into a single map with placeholder values.

src/material/form-field/BUILD.bazel

@@ -52,12 +52,11 @@ sass_binary(
 sass_library(
     name = "form_field_partials",
     srcs = [
-        "_form-field-density.scss",
         "_form-field-focus-overlay.scss",
         "_form-field-high-contrast.scss",
         "_form-field-native-select.scss",
-        "_form-field-sizing.scss",
         "_form-field-subscript.scss",
+        "_mdc-text-field-density-overrides.scss",
         "_mdc-text-field-structure-overrides.scss",
         "_mdc-text-field-textarea-overrides.scss",
         "_user-agent-overrides.scss",

src/material/form-field/_form-field-density.scss

@@ -3,7 +3,6 @@
 
 @use '../core/tokens/m2/mat/form-field' as tokens-mat-form-field;
 @use '../core/tokens/token-utils';
-@use './form-field-sizing';
 
 @mixin private-form-field-subscript() {
   // Wrapper for the hints and error messages.
@@ -50,7 +49,7 @@
 
   // Spacer used to make sure start and end hints have enough space between them.
   .mat-mdc-form-field-hint-spacer {
-    flex: 1 0 form-field-sizing.$mat-form-field-hint-min-space;
+    flex: 1 0 1em;
   }
 
   // Single error message displayed beneath the form field underline.

src/material/form-field/_form-field-sizing.scss

@@ -9,7 +9,6 @@
 @use '../core/typography/typography';
 @use '../core/style/sass-utils';
 @use '../core/tokens/token-utils';
-@use './form-field-density';
 
 @mixin base($theme) {
   @if inspection.get-theme-version($theme) == 1 {
@@ -85,7 +84,10 @@
     @include _theme-from-tokens(inspection.get-theme-tokens($theme, density));
   }
   @else {
-    @include form-field-density.private-form-field-density($theme);
+    @include sass-utils.current-selector-or-root() {
+      @include token-utils.create-token-values(tokens-mat-form-field.$prefix,
+        tokens-mat-form-field.get-density-tokens($theme));
+    }
   }
 }
 

src/material/form-field/_form-field-subscript.scss

@@ -0,0 +1,61 @@
+@use 'sass:list';
+@use '@material/textfield' as mdc-textfield;
+@use '../core/tokens/m2/mat/form-field' as tokens-mat-form-field;
+@use '../core/tokens/token-utils';
+
+// Mixin that includes the density styles for form fields. MDC provides their own density
+// styles for MDC text-field which we cannot use. MDC relies on input elements to stretch
+// vertically when the height is reduced as per density scale. This doesn't work for our
+// form field since we support custom form field controls without a fixed height. Instead, we
+// provide spacing that makes arbitrary controls align as specified in the Material Design
+// specification. In order to support density, we need to adjust the vertical spacing to be
+// based on the density scale.
+@mixin private-text-field-density-overrides() {
+  @include token-utils.use-tokens(
+    tokens-mat-form-field.$prefix, tokens-mat-form-field.get-token-slots()) {
+    $height: token-utils.get-token-variable(container-height);
+
+    .mat-mdc-form-field-infix {
+      // We add a minimum height to the infix container to ensure that custom controls have the
+      // same default vertical space as text-field inputs (with respect to the vertical padding).
+      min-height: var(#{$height});
+
+      @include token-utils.create-token-slot(padding-top,
+        filled-with-label-container-padding-top);
+      @include token-utils.create-token-slot(padding-bottom,
+        filled-with-label-container-padding-bottom);
+
+      .mdc-text-field--outlined &,
+      .mdc-text-field--no-label & {
+        @include token-utils.create-token-slot(padding-top, container-vertical-padding);
+        @include token-utils.create-token-slot(padding-bottom, container-vertical-padding);
+      }
+    }
+
+    // By default, MDC aligns the label using percentage. This will be overwritten based
+    // on whether a textarea is used. This is not possible in our implementation of the
+    // form-field because we do not know what type of form-field control is set up. Hence
+    // we always use a fixed position for the label. This does not have any implications.
+    .mat-mdc-text-field-wrapper .mat-mdc-form-field-flex .mat-mdc-floating-label {
+      top: calc(var(#{$height}) / 2);
+    }
+
+    // We need to conditionally hide the floating label based on the height of the form field.
+    .mdc-text-field--filled .mat-mdc-floating-label {
+      display: var(#{token-utils.get-token-variable(filled-label-display)}, block);
+    }
+
+    // For the outline appearance, we re-create the active floating label transform. This is
+    // necessary because the transform for docked floating labels can be updated to account for
+    // the width of prefix container.
+    .mat-mdc-text-field-wrapper.mdc-text-field--outlined .mdc-notched-outline--upgraded
+    .mdc-floating-label--float-above {
+      // Needs to be in a string form to work around an internal check that incorrectly flags this
+      // interpolation in `calc` as unnecessary. If we don't have it, Sass won't evaluate it.
+      $translate: 'calc(#{mdc-textfield.get-outlined-label-position-y(var(#{$height}))} * -1)';
+      --mat-mdc-form-field-label-transform: translateY(#{$translate})
+        scale(var(--mat-mdc-form-field-floating-label-scale, 0.75));
+      transform: var(--mat-mdc-form-field-label-transform);
+    }
+  }
+}